The amount of output sag is also a function of the maxi-

mum duty factor, which can be calculated from the on-

time and minimum off-time:

where

and minimum off-time = 400ns (typ) (see Table 5 for K

values).

The amount of overshoot during a full-load to no-load

transient due to stored inductor energy can be calculated

as:

Setting the Current Limit

For most applications, set the MAX1844 current limit by

the following procedure:

1) Determine the minimum (valley) inductor current

large, and load current is maximum. The minimum

rent (Figure 4).

2) The sense resistor determines the achievable

current-limit accuracy. There is a trade-off between

current-limit accuracy and sense-resistor power dis-

sipation. Most applications employ a current-sense

voltage of 50mV to 100mV. Choose a sense resistor

so that:

Extremely cost-sensitive applications that do not

require high-accuracy current sensing can use the on-

resistance of the low-side MOSFET switch in place of

the sense resistor by connecting CS to LX (Figure 8b).

Q2 data sheet, and add a margin of 0.5%/°C for the

mine the CS threshold voltage. If the default 100mV

threshold is unacceptable, set the value as in step 2

above.

In all cases, ensure an acceptable CS threshold volt-

age despite inaccuracies in resistor values.

Output Capacitor Selection

The output filter capacitor must have low enough effective

series resistance (ESR) to meet output ripple and load-

transient requirements, yet have high enough ESR to sat-

isfy stability requirements.

For CPU core voltage converters and other applications

where the output is subject to violent load transients, the

output capacitor’s size depends on how much ESR is

needed to prevent the output from dipping too low under

a load transient. Ignoring the sag due to finite capaci-

tance:

In non-CPU applications, the output capacitor’s size often

depends on how much ESR is needed to maintain an

acceptable level of output voltage ripple:

The actual microfarad capacitance value required relates

to the physical size needed to achieve low ESR, as well

as to the chemistry of the capacitor technology. Thus, the

capacitor is usually selected by ESR and voltage rating

rather than by capacitance value (this is true of tantalums,

OS-CONs, and other electrolytics).

When using low-capacity filter capacitors, such as

ceramic or polymer types, capacitor size is usually deter-

Generally, once enough capacitance is added to meet

the overshoot requirement, undershoot at the rising load

R

V

LIR I

ESR

P- P

LOAD(MAX)

≤

×

R

V

I

ESR

DIP

LOAD(MAX)

≤

V

LI

CV

SOAR

LOAD MAX

OUT OUT

≈

×∆

()

()

2

2

DUTY

K (V

+ 0.075V)/ V

K (V

+ 0.075V)/ V

+ min off - time

OUT

IN

OUT

OUT

=

V

(I

)

L

2

C

DUTY (V

- V

)

SAG

LOAD(MAX)

2

OUT

IN(MIN)

OUT

=

×

××

∆

High-Speed Step-Down Controller with

Accurate Current Limit for Notebook Computers

______________________________________________________________________________________

17

DL

CS

LX

a)

b)

MAX1844

DL

CS

LX

MAX1844

Figure 8. Current-Sense Circuits